1. Field of the Invention
The present invention relates to a counterbalance mechanism for use in an internal combustion engine.
2. Description of the Relevant Art
Counterbalance mechanisms for reducing the vibrations of some internal combustion engines while the engines are in operation include a primary counterweight which is rotated at the same speed as and in the opposite direction to the crankshaft so as to reduce a primary vibration. The counterbalance mechanisms may also include a secondary counterweight which is rotated at a speed twice that of the crankshaft for lowering a secondary vibration.
One counterbalance mechanism, particularly one having a primary counterweight, is disclosed in Japanese Laid-Open Patent Publication No. 58-39863, for example. The primary counterweight is rotatably supported on the main shaft of a transmission which is operatively coupled to the crankshaft of an engine through a clutch. The primary counterweight has an integral driven sprocket operatively connected through a chain to a drive sprocket integral with the crankshaft. Therefore, the primary counterweight can be rotated in synchronism with the crankshaft.
According to the disclosed counterbalance mechanism, however, driven sprockets dedicated for rotating the counterweight are mounted on the main shaft and countershaft, respectively, and also the drive sprocket dedicated for rotating the counterweight is mounted on the crankshaft. The counterbalance mechanism of such a construction has been an obstacle to efforts to make the internal combustion engine smaller in size and lighter in weight.
Since the drive and driven sprockets and the chain trained therearound are required for rotating the counterweight, the number of components making up the internal combustion engine is large, and the number of steps of assembling the engine is also large. The internal combustion engine is therefore costly to manufacture.